In selective call networks, such as a paging network, a sequence of digital symbols is modulated and sent from a transmitter to a receiver. The receiver performs the necessary signal processing functions to demodulate the signal and to decode the demodulated signal into digital symbols. In order to decode the received signal into digital symbols, the receiver must obtain timing and other synchronization information from the received signal upon which it can base its symbol decoding decisions. Once synchronization onto a given frame of a received signal has been achieved, a tracking element is utilized to adapt thresholds that are used for symbol decoding as well as for the timing recovery operation.
In known pagers utilizing such protocols as FLEX having a four-level structure with a 6400 bps frame, the output of an FM demodulator of the pager is sampled at a very high rate. Peak and valley techniques are applied to the sampled output of the demodulator in order to establish symbol detection thresholds. A zero-crossing phase-locked loop is utilized for acquiring and maintaining timing synchronization. During four-level data detection, however, the performance of the receiver is very sensitive to the quality of the symbol detection thresholds that are set by the peak and valley routine. Depending on the distribution of the data, the peak and valley search operation will result in sub-optimum detection thresholds so that on average performance degrades. Further, the phase-locked loop clock recovery system must run at a very high sampling rate and can experience timing phase jitter even in the presence of a strong received signal. This jitter typically occurs about the center of the zero-crossings of the waveform so that the loop does not necessarily lock onto the point of the sampled waveform where inter-symbol interference is minimum, this point being known as the point of maximum-eye-opening.
Stochastic gradient symbol recovery routines have been used in radios, but it has not been known to use these routines in selective call receiving devices such as a pager. A stochastic gradient symbol recovery routine or system will lock onto the maximum-eye-opening point with very little jitter and further requires only a few samples of the signal per symbol. However, stochastic gradient systems can experience false lock points during acquisition that render the system unusable. One method for acquiring initial acquisition for a stochastic gradient symbol recovery system is to use a signal correlator that correlates the raw received signal with an expected synchronization word waveform. This method, however, has severe falsing problems in the presence of noise.